a. Field of the Invention
This invention pertains to audio signal processing devices. More particularly this invention pertains to devices used to increase the amount of energy contained in the lower or bass frequencies of an audio signal relative to the higher frequency components without overloading any subsequent audio amplifiers.
b. Description of the Prior Art
Several methods have been used in the prior art to increase the energy contained in the lower or bass frequencies in an audio signal relative to the higher frequency components. For instance, a set of band-pass filters, each having an adjustable attenuation, has been used to emphasize or de-emphasize the frequency components within each band of frequencies relative to the components within the other bands. If the attenuation of the low frequency band-pass filter is adjusted to be less than the attenuation of the higher frequency band-pass filters, the low frequency components are enhanced relative to the higher frequency components. A single filter that exhibits a lower attenuation for low frequency components relative to its attenuation for higher frequency components may also be used for this purpose. However, because the peak level of the audio input signal is typically primarily determined by the peak level of the low frequency components, a simple "pre-emphasis" of the low frequency components often results in the overloading of any subsequent audio amplifiers.
Multiband compression has also been used to emphasize the bass components. A multiband compression system splits the audio signal into separate bands of frequency components and then separately compresses the amplitude range of the signals within each of these bands.
The energy in the bass components relative to the higher frequencies has also been increased by combining the input audio signal with subharmonics of the input audio signal. This method, however, very often substantially increases the peak level of the signal.